Chlorine has disinfected water supplies for hundreds of years. Such water is not generally pure; the water may contain harmful bacteria; innocuous proteins such as from decaying leaves; chlorine-reactive ions of iron, sulfide, and nitrite; and hard-water compounds such as magnesium or calcium carbonate. As community water supplies have expanded in scale, accurate and reliable automated chlorination control systems have become preferred. Underchlorination runs the risk of disease; overchlorinated water is distasteful and reacts to natural organic matter (NOM) to form chlorinated organic matter (also known as trihalomethanes, or THM). Recently, the U.S. Environmental Protection Agency has reduced allowable concentrations of THMs, which are known carcinogens.
Chlorine is typically added to water in elemental form as Cl2. Chlorine in water is quickly converted to hypochlorous acid which dissociates to H+ and OCl−.Cl2+H2O< - - - >HCl+HOCl< - - - >2H++OCl−+Cl−  (Eq. 1)Chlorine may also be added in other forms, such as calcium hypochlorite. The unmodified term “chlorine” is used to encompass Cl2, HOCl, or OCl−. By adding ammonia as well as chlorine, monochloramine (NH2Cl) is the desired result; it is a less aggressive oxidizer less likely to attack metals such as structural steel and toxic lead in the distribution pipes, is longer-lasting, and less reactive with NOM.HOCl+NH3< - - - >NH2Cl(monochloramine)+H2O  (Eq. 2)The unmodified term “ammonia” is used to encompass NH3 or NH4+, the ammonium ion resulting from hydrolysis. Monochloramine has about 25 times the disinfecting power of the OCl− ion due in part to its neutral charge which more easily penetrates bacterial cell walls. But a simple addition of a 1:1 molar ratio of chlorine and ammonia is not optimal; variables such as chlorine consumed by water impurities such as iron, pH, temperature, natural aqueous ammonia, and contact time all affect the optimal amount of chlorine, and therefore the desired chlorine: ammonia feed ratio. If ammonia is overfed, excess nitrogen is a food source for undesirable bacteria in the water distribution system. If ammonia is underfed, dichloramine or at higher Cl:N ratios, even trichloramine is produced from monochloramine:HOCl+NH2Cl< - - - >NHCl2(dichloramine)+H2O  (Eq. 3)2NH2Cl(acid-catalyzed)< - - - >NHCl2+NH3  (Eq. 4)HOCl+NHCl2< - - - >NCl3(trichloramine)+H2O  (Eq. 5)Trichloramine is less commonly but more accurately known as nitrogen trichloride. The higher chloramines, dichloramine and trichloramine, have unacceptable pungent taste and odor and are less effective disinfectants. The chlorine in uncombined form or monochloramine is known as available or ‘residual’ chlorine. Chlorine, hypochlorous acid, hypochlorite ion, monochloramine, dichloramine, and trichloramine are collectively referred to as chlorination compounds. The acid-catalyzed reaction of monochloramine with itself (Eq. 4) to form dichloramine and ammonia, is known as a disproportionation reaction.
In a control system, a desirable control signal is one which linearly achieves a low level to indicate the controlled variable should be lowered, and a high level to indicate the controlled variable should be raised. But chlorination is not properly controlled by simply measuring the resultant chlorine concentration. A major problem is that available chlorine concentration does not rise monotonically as more chlorine is added. (For clarity, the discussion of chlorine:ammonia ratios in this and the next three paragraphs will ignore the variables of consumption, pH, temperature, and contact time, which affect the actual optimal ratios.) When the molar chlorine:ammonia ratio exceeds 1:1, the available chlorine concentration actually decreases, due to competing reactions forming higher chloramines, dropping by over 75%, until a chlorine:ammonia ratio of about 1.7:1 is reached. Above this ratio, saturation occurs and available chlorine concentration rises again. A similar problem occurs if the monochloramine concentration is monitored; at higher chlorine:ammonia ratios, monochloramine concentration actually decreases due to conversion to dichloramine and trichloramine. Neither measuring available chlorine, or monochloramine, provides a linear or even monotonic control indicator over the desired range.
Conventional strategies for controlling the chlorine:ammonia ratio fall into two common methods:
1. A very small ammonia concentration is maintained to ensure that monochloramine is the predominant chlorine species. However in practicality, control of ammonia at levels on the range of tens of parts per billion is difficult, nearing the detection limit of the analyzer. An excess ammonia concentration must be maintained, simply to stay in the detection range of the analyzer.
2. The chlorine:ammonia is controlled to a molar 1:1 ratio or below. If correctly done, the predominant chlorine species is monochloramine. Unfortunately, aqueous ammonia already present, or ions consuming chlorine, alter this ratio. If chlorine of overfed, both available chlorine and ammonia decrease and not at the same rate, leading to incorrect control decisions. Conventional analyzers for this method measure monochloramine concentration. But monochloramine concentration actually decreases as the chlorine:ammonia ratio exceeds 1:1, again leading to incorrect control decisions.
U.S. Pat. No. 6,315,950, issued Nov. 13, 2001 to Harp et al., discloses a system for the measurement of monochloramine, and Cl:NH3 control to a molar 1:1 ratio or below. Monochloramine is reacted with phenol compounds, using a nitroferricyanide catalyst, to form indophenols, which are detected by their absorbance in the 600-800 nanometer (nM) range. Multiple analyzer cells are used to measure free ammonia, total ammonia, and monochloramine. The process uses toxic chemicals, and produces toxic waste. In practice, the analysis time of about 20 minutes, is excessive for many control system applications.
Multiple analyzer cells add cost to an analysis system, and also exhibit differential drift. If one cell has a sensitivity gain, the resultant ratios of its measurements compared to the results of another cell, are artificially inflated. Calibration and nulling become onerous, or accuracy suffers. If a single cell is utilized, a sensitivity gain yields an increase in both measurements, which tends to null out comparison errors.
A water-analysis system may employ ultraviolet (UV) spectroscopy. With two exceptions in the infrared, water is transparent to wavelengths from 200 to 1400 nM. But some dissolved chemicals will absorb certain wavelengths; each chemical has a particular absorbance pattern, or signature, whose strength is proportional to its concentration. With repeatable illumination and spectrum collections of a sample, a blank spectrum is taken; a chemical added to initiate a reaction resulting in a product; the product spectrum is taken; the blank spectrum subtracted from the product spectrum; and the difference spectrum is analyzed to determine the product's identity and concentration. The spectrum is produced by a monochromator, which may operate through prismatic, diffractive, holographic, optical filtering, or other means. Analysis of more spectrum points generally provides greater accuracy and better isolation from extraneous absorbance generated by various contaminants. Subtraction of spectra is performed in accordance with Beer's Law; optical absorbances are subtracted, where absorbance is defined as the negative log of the light intensity.
UV spectrometers used for such analysis typically include a UV light source; an analyzer cell directly or indirectly illuminated by the UV light, and plumbed to the sample water and various sources of liquid reagents; a UV light collection mechanism which receives the UV light transmitted through the sample; a monochromator to produce and focus the UV spectrum; an array detector which produces an electrical signal representative of the spectrum; an analog-to-digital converter which converts the electrical signal to digital data; and a microprocessor which performs math, control, and logic operations, including calculation of chemical concentrations, sequential operation of the analyzer hardware, control of external valves, and communications to operators, recorders, or other systems.
Some chemicals, such as ammonia, do not have reliable absorbance characteristics. To be measured with spectroscopy, these chemicals must be converted to another chemical with a reliable spectrum.
There is a need for a water chlorination control system which is capable of measurement in the desired range; exhibits accuracy and stability over a wide range of chlorine:ammonia ratios; maintains a minimum concentration of both ammonia and dichloramine; may also maintain a desired amount of monochloramine; requires no toxic-chemical handling; uses a minimum of analyzer cells, preferably a single analyzer cell; and has sufficient speed to operate in a control environment.